1. Field of the Invention
The invention relates to a method for the determination of a significant parameter of the progress of an extracorporeal treatment of blood, in particular of a purification treatment intended to palliate renal insufficiency, such as haemodialysis or haemodiafiltration.
2. Description of the Related Art
Haemodialysis consists of circulating, on both sides of the semipermeable membrane of an exchanger, the blood of a patient or a treatment fluid which is substantially isotonic to blood, so that, during the diffusive transfer which is established across the membrane for the substances having different concentrations on either side of the membrane, the blood impurities (urea, creatinine and the like) migrate from the blood towards the treatment fluid. The electrolytic concentration of the treatment fluid is also generally chosen in order to correct the electrolytic concentration of the patient's blood.
In a haemodiafiltration treatment, to the diffusive transfer obtained by dialysis is added a convective transfer by ultrafiltration resulting from a positive pressure difference created between the blood side and the treatment fluid side of the membrane.
It is of greatest interest to be able to determine, throughout a treatment session, one or more significant parameters of the progress of the treatment in order to be able, where appropriate, to modify the treatment conditions as initially set for the purpose of a determined therapeutic objective.
The parameters of which knowledge makes it possible to monitor the progress of the treatment, that is to say also to evaluate the correspondence between the treatment conditions initially set and the therapeutic objective, are in particular the blood concentration of a given solute (for example sodium), the real dialysance or the real clearance of the exchanger for such solute (the dialysance and the clearance representing the purification efficiency of the exchanger), or the dose of dialysis administered after a treatment time t, which, according to the work of Sargent and Gotch, can be assimilated to the dimensionless ratio Kt/V, where K is the real clearance for urea, t the treatment time elapsed, and V the volume of distribution of urea, that is to say the total volume of water of the patient (Gotch FA, Sargent SA. A mechanistic analysis of the National Cooperative Dialysis Study (NCDS). Kidney int 1985; 28: 526-34).
All these parameters pose the same problem for their determination, which is to necessitate the precise knowledge of a physical or chemical characteristic of the blood, whereas this characteristic cannot, in practice, be obtained by direct measurement on a sample for therapeutic, prophylactic and financial reasons: On the one hand, it is out of the question to take from a patient, who is often anaemic, multiple samples which would be necessary in order to monitor the efficiency of the treatment as it progresses. On the other hand, given the risks connected with the handling of possibly contaminated blood samples, the general tendency is to avoid such handling. Finally, the laboratory analysis of a blood sample is both costly and relatively long, which is incompatible with the desired objective.
The document EP 0 547 025 describes a method for the in vivo determination of the haemodialysis parameters which do not necessitate the carrying-out of measurements on blood. According to this method, whose implementation requires means for regulating the ionic concentration of the dialysis fluid and means for measuring the sodium concentration of the dialysis fluid or its conductivity, the electrolytic transfer of the dialysis fluid is measured at two different predetermined concentrations of the dialysis fluid and the dialysance is deduced therefrom.
This method necessitates exposing the patient to a dialysis fluid differing substantially from the dialysis fluid prescribed during the time necessary for the stabilization of the concentration of the dialysis fluid downstream of the exchanger, failing which the measurement applies to a dialysis fluid of intermediate concentration, and all the calculations subsequently made from this measurement are erroneous.